The broadsword, the battle-axe and the spear could be two handed only to constraints the player to choose between attack power and defense with shield.
These constraints could be configured in a configuration file to match playstyle.

Copied from #40
Two-handed weapons could outright disable usage of items in the offhand slot when held. Holding one would hide both the slot and the hand from the player's screen, as well as the offhand item from the third person model. Saying "no offhand item can be used with this weapon that uses two hands" makes more sense to me than "only shields cannot be used with a two-handed weapon," and also answers the question of where the offhand item goes: it stays in the slot, but isn't useable.
This would also prevent the usage of, say, the Totem of Undying with these weapons, for better or worse.